1. Field of the Invention
This invention relates to a single and three phase heater clement circuit for an electrical heating unit and more particularly to an improvement in the construction and arrangement of a heater element circuit for a ceramic fiber for preventing short circuiting of the heater.
2. Description of the Prior Art
U.S. Pat. No. 4,575,619 discloses a ceramic fiber heater having a plurality of serpentine heater elements arranged side by side. FIG. 3 is a plane view of a conventional ceramic fiber heater having a plurality of serpentine heater elements to be connected across a single-phase electric power source. In FIG. 3, reference numerals 1a, 1b, - - - designate serpentine heater elements, respectively.
In such a ceramic fiber heater, four heater elements 1a-1d, for example, are arranged side by side on a base made of an insulation material to form one block 3a of the heater elements. In said one block 3a, one end 4a of a first heater element 1a is connected to one of terminals of a single-phase electric power source of 200 V, for example, and the other end of said first heater element 1a is connected through a connecting line 2 with one end of a second heater element 1b, adjacent to said other end of said first heater element 1a. Similarly, the other end of the second heater element 1b is connected through a connecting line 2 with one end of a third heater element 1c, and the other end of the third heater element 1c is connected through a connecting line 2 with one end of a fourth heater element 1d, so that the first to fourth heater elements 1a-1d form a series circuit. Both terminals 4a of said series circuit is connected across the single-phase electric power source of 200 V.
A block 3b similar to said block 3a is arranged adjacent to said block 3a on said base.
FIG. 4 shows a plane view of a conventional ceramic fiber heater to be connected across a three-phase electric power source of 200 V, for example. In this ceramic fiber heater, as like as the embodiment shown in FIG. 3, four heater elements 1a-1d, for example, are arranged side by side on a base made of insulation material to form one block 3a. In said one block 3a, the four heater elements 1a-1d are connected in series. Blocks 3b and 3c, each similar to said block 3a, are arranged side by side on the base, and each series circuit of each block 1a-1d is applied with a phase voltage of the three-phase electric power source of 200 V.
As shown in FIG. 3, both ends 4a of the series circuit in the block 3a are connected across the single-phase electric power source of 200 V. With respect to the block 3b, similarly, both ends 4b of the series circuit are connected across the single-phase electric power source of 200 V.
Accordingly, a voltage difference of 200 V is formed between one end 4a of the series circuit in the block 3a and one end 4b of the series circuit in the block 3b, adjacent to said one end 4a. Normally, a distance between said ends 4a and 4b is short, such as several millimeters, and the insulation resistance or specific resistance of the ceramic fiber base made of alumina silica, etc. is reduced remarkably at a temperature higher than about 1,000.degree. C., so that a leakage current flows through the insulation material between the two ends 4a and 4b and the leakage current becomes large due to the voltage applied between said two ends 4a and 4b, and the heater element is liable to be burned and damaged.
Specifically, in case of mullite (compound of alumina and silica), for example,
at 900.degree. C., the specific resistance is 2.times.10.sup.5 (.OMEGA..multidot.cm), PA1 at 1000.degree. C., the specific resistance reduces to 5.5.times.10.sup.4 .OMEGA.cm, PA1 at 1100.degree. C., the specific resistance reduces to 1.2.times.10.sup.4 .OMEGA.cm, and PA1 at 1200.degree. C., the specific resistance reduces to 2.5.times.10.sup.3 .OMEGA.cm.
A voltage of the power source is applied between said adjacent ends 4a and 4b of the series circuits, so that a leakage current flows through the electrical insulation material therebetween at a higher temperature and accordingly the temperature of said material portion is elevated.
As a result, the insulation resistance of said material portion is reduced, so that the leakage current is increased. Such phenomenon is repeated and accordingly the temperature of said material portion is elevated abnormally, so that the heater element is burned and damaged.
In case that the three-phase electric power source is used, as shown in FIG. 4, a voltage difference of about 200 V is formed between one end 5a of the series circuit in the block 3a and one end 5b of the series circuit in the block 3b, adjacent to said one end 5a or between the other end 5b of the series circuit in the block 3b and one end 5c of the series circuit in the block 3c, adjacent to said other end 5b, so that similar defects and problems often arise.